Audio amplifier circuit having inputs for three audio channels

ABSTRACT

An audio amplifier circuit that is to receive three audio channel signals as inputs. A first amplifier has a first pair of outputs one of which produces a first combination sum of the three audio channel signals, and the other produces a second, different combination sum of the three audio channel signals. A second amplifier has a second pair of outputs one of which produces a third, different combination sum of the three audio channel signals and the other produces a fourth, different combination sum of the three audio channel signals. Other embodiments are also described and claimed.

FIELD

An embodiment of the invention relates generally to multi-channel audiopower amplifiers and more particularly to audio power amplifiers thatdrive three or more speakers in accordance with three or more audiochannels, respectively, as input. Other embodiments are also described.

BACKGROUND

Multi-channel audio power amplifiers that can drive two or morespeakers, each in accordance with a respective audio channel, arecommonplace. Audio amplifiers whose output configuration supports abridge tied load (BTL) are also advantageous for a variety of reasons. Adifficulty that arises however in the context of small form factorconsumer electronic devices, such as desktop computers, laptopcomputers, and tablet computers, is that a class D, multi-channel audioamplifier that can drive three input audio channels into three speakersin BTL configurations has a large power transistor count, making itdifficult to integrate into a small housing. That task can be met by abrute force technique of three, class D full bridge amplifiers (eachhaving a BTL output), however such a technique may require too manypower transistors which would take up too much space and would be costprohibitive.

A class D audio power amplifier product is available that has fouroutput nodes, and is programmable into several different outputconfigurations, namely a) 2 channels of BTL outputs (driving twospeakers), b) 4 channels of single ended outputs (driving fourspeakers), and c) 2 channels of single ended outputs and 1 channel in aBTL configuration (driving a total of three speakers). That producthowever does not support an arrangement where three channels can bedriven through three BTL outputs into three speakers, respectively.

SUMMARY

An embodiment of the invention is an audio amplifier circuit that is toreceive three audio channel signals as input. A first amplifier has afirst pair of outputs, wherein one of the first pair of outputs is toproduce a first combination sum of the three audio channel signals, andthe other produces a second, different combination sum of the threeaudio channel signals. A second amplifier has a second pair of outputs,wherein one of the second pair of outputs produces a third, differentcombination sum of the three audio channel signals and the otherproduces a fourth, different combination sum of the three audio channelsignals. Such an audio amplifier circuit may be used to drive threespeakers through a separate BTL output pair for each of the threespeakers. Each of the first and second amplifiers may be viewed ashaving two output legs (yielding a combined four legs of voltage drive).By independently controlling the voltage on each leg of the twoamplifiers, three input audio signals can be delivered to three loads,respectively, each in the form of a differential signal across a pair ofinputs of its respective load.

The above summary does not include an exhaustive list of all aspects ofthe present invention. It is contemplated that the invention includesall systems and methods that can be practiced from all suitablecombinations of the various aspects summarized above, as well as thosedisclosed in the Detailed Description below and particularly pointed outin the claims filed with the application. Such combinations haveparticular advantages not specifically recited in the above summary.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention are illustrated by way of example andnot by way of limitation in the figures of the accompanying drawings inwhich like references indicate similar elements. It should be noted thatreferences to “an” or “one” embodiment of the invention in thisdisclosure are not necessarily to the same embodiment, and they mean atleast one. Also, a given figure may be used to illustrate the featuresof more than one embodiment of the invention, and not all elements inthe figure may be required for a given embodiment.

FIG. 1 is a combined block diagram and circuit schematic of anembodiment of an audio amplifier circuit having inputs to receive threeaudio channels, respectively.

FIG. 2 illustrates another configuration of the audio amplifier circuitof FIG. 1.

FIG. 3a illustrates yet another configuration of the audio amplifiercircuit of FIG. 1

FIG. 3b illustrates yet another configuration of the audio amplifiercircuit of FIG. 1.

FIG. 4 is a combined block diagram and circuit schematic of the audioamplifier circuit implemented using four half-bridge amplifiers.

FIG. 5 is a combined block diagram and circuit schematic of an exampleclass D half-bridge output stage.

FIG. 6 is a block diagram of relevant parts of an example portableelectronic audio device in which the audio amplifier circuit can beimplemented.

DETAILED DESCRIPTION

Several embodiments of the invention with reference to the appendeddrawings are now explained. Whenever aspects of the embodimentsdescribed here are not clearly defined, the scope of the invention isnot limited only to the parts shown, which are meant merely for thepurpose of illustration. Also, while numerous details are set forth, itis understood that some embodiments of the invention may be practicedwithout these details. In other instances, well-known circuits,structures, and techniques have not been shown in detail so as not toobscure the understanding of this description.

FIG. 1 is a combined block diagram and circuit schematic of anembodiment of an audio amplifier circuit 4 having as inputs three audiochannels or signals x1, x2 and x3. All of these audio channels may beindependently controlled by an audio processor (not shown) that isupstream of the inputs, e.g. a digital audio processor that decoded thechannels x1, x2 and x3 as part of an encoded, multiple audio channelmultimedia stream or stored file—see FIG. 6 for an example. In oneembodiment, x3 may be a center channel while x1 and x2 are left andright channels, respectively. The center channel x3 could have beenderived from the left and right channels or it could have been generatedindependently of them (by the upstream digital audio processor, forexample).

There is a first amplifier 5 having a first pair of outputs, wherein oneof the first pair of outputs is to produce a first combination sum(e.g., x1 +x2 +x3) of the three audio channel signals that are providedas inputs of the first amplifier 5. The other one of the first pair ofoutputs produces a second, different combination sum (e.g., −x1+x2+x3)of the three audio channel signals. There is also a second amplifier 6that has a second pair of outputs, wherein one of the second pair ofoutputs produces a third, different combination sum (e.g., x1+x2−x3) ofthe three audio channel signals, and the other produces a fourth,different combination sum (e.g., x1−x2−x3) of the three audio channelsignals. The amplifiers 5, 6 are power amplifiers that in this exampleare coupled to drive three independent speakers 1, 3, and 2 in such away that each of two outputs (from the combined four outputs of thefirst and second amplifiers 5, 6) is shared by inputs of two of thefirst, second and third speakers 1, 2, 3. In FIG. 1, the upper output ofthe amplifier 5 is shared by i) the upper input of the speaker 3 and ii)the upper input of the speaker 1, and the upper output of the amplifier6 is shared by i) the lower input of the speaker 3 and ii) the upperinput of the speaker 2. Each of the first and second amplifiers 5, 6 mayhave a full bridge output stage that is coupled to drive its floatingload (as shown). An example is a class D bridge tied load (BTL) outputstage. Each of the speakers 1, 3, 2 may be an electro-dynamicloudspeaker having a respective pair of drive inputs as shown (which maybe the two terminals of a driver voice coil of the loudspeaker).

The three signals, x1(t), x2(t), and x3(t), may be viewed as being mixedtogether, in the time domain, in different combinations, on each of thefour legs of the first and second amplifiers 5, 6. In other words, thefirst, second and third audio channel signals have been embedded intoeach of the four amplifier output legs, in different combinations. Inone embodiment, referring now to FIG. 1, the polarity of x1, x2 and x3on each output leg of the amplifiers 5, 6 is controlled so as tosimultaneously achieve the following three conditions. First, x1(t), andnot x2(t) and not x3(t), appears as a differential signal across thepair of input terminals of the speaker 1 (scaled by a factor of two),while x1 is presented common mode at the pair of input terminals of thespeaker 2, and at the pair of input terminals of the speaker 3.Similarly, x2(t), and not x1(t) and not x3(t), appears as a differentialsignal across the pair of input terminals of the speaker 2, while x2 ispresented common mode at the pair of input terminals of the speaker 1,and at the pair of input terminals of the speaker 3. Finally, x3(t), andnot x1(t) and not x2(t), appears as a differential signal across thepair of input terminals of the speaker 3, while x3 is presented commonmode at the pair of input terminals of the speaker 1, and at the pair ofinput terminals of the speaker 2. In this configuration, each of thethree speakers 1, 3, and 2 is being simultaneously driven by itsrespective audio channel x1, x3 and x2 (with a scaling factor of 2). Thesolution depicted in FIG. 1 for driving the three speakers 1, 3, and 2(with their respective audio channels signals x1, x3, and x2, which mayif desired be produced independent of each other) is expected to requirea smaller power transistor count than the brute force technique ofhaving a separate full bridge (differential output) amplifier coupled todrive the pair of input terminals of each of the speakers 1, 3, 2. Stillreferring to FIG. 1, in this particular configuration of the audioamplifier circuit 4 in which an amplifier output leg is shared by theinputs of two different speakers, the first pair of outputs of the firstamplifier 5 is coupled to drive the respective pair of inputs of thefirst speaker 1, respectively. The second pair of outputs of the secondamplifier 6 is coupled to drive the respective pair of inputs of thesecond speaker, respectively. Lastly, a) one of the respective pair ofinputs of the third speaker 3 is coupled to be driven by one of thefirst pair of outputs (of the first amplifier 5), and b) the other oneof the respective pair of inputs of the third speaker 3 is coupled to bedriven by one of the second pair of outputs (of the second amplifier 6).

Several different configurations of the audio amplifier circuit 4 arepossible, in which the coupling of the output legs of the amplifiers 5,6 to the speakers 1, 2, 3 and the assignment of combinations of theinput audio channels are different than shown in FIG. 1, while stillproducing the same sound output shown in FIG. 1 (from the speakers 1, 2,3.) The audio amplifier circuit 4 still achieves the same result of eachspeaker 1, 2, 3 being simultaneously driven by its respective audiochannel x1, x2, x3. For example, FIG. 2 shows a configuration where thecombination of audio channels appearing on some of the output legs aredifferent than in FIG. 1, but the resulting sound being outputted by thespeakers is the same as in FIG. 1, because the coupling (wiring) of theinput terminals of the speaker 3 is changed as shown in FIG. 2 (so thatthe input terminals of the speaker 3 are coupled to a different pair ofthe output legs than in FIG. 1). As seen in FIG. 2, the amplifiers 5, 6are coupled to drive the three independent speakers 1, 3, and 2 in sucha way that each of two outputs (from the combined four outputs of thefirst and second amplifiers 5, 6) is shared by inputs of two of thefirst, second and third speakers 1, 2, 3. In FIG. 2, the upper output ofthe amplifier 5 is shared by i) the upper input of the speaker 3 and ii)the upper input of the speaker 1, and the lower output of the amplifier6 is shared by i) the lower input of the speaker 3 and ii) the lowerinput of the speaker 2.

Another configuration is shown in FIG. 3a , where in this case thepolarity of the input terminals of the speaker 1 as coupled to the firstamplifier 5 is reversed (relative to that of FIG. 1 and FIG. 2) but thesound output remains the same. As seen in FIG. 3a , the amplifiers 5, 6are coupled to drive the three independent speakers 1, 3, and 2 in sucha way that each of two outputs (from the combined four outputs of thefirst and second amplifiers 5, 6) is shared by inputs of two of thefirst, second and third speakers 1, 2, 3. In FIG. 3a , the upper outputof the amplifier 5 is shared by i) the upper input of the speaker 3 andii) the lower input of the speaker 1, and the lower output of theamplifier 6 is shared by i) the lower input of the speaker 3 and ii) thelower input of the speaker 2.

Yet another configuration is shown in FIG. 3b . As seen in FIG. 3b , theamplifiers 5, 6 are coupled to drive the three independent speakers 1,3, and 2 in such a way that each of two outputs (from the combined fouroutputs of the first and second amplifiers 5, 6) is shared by inputs oftwo of the first, second and third speakers 1, 2, 3. In FIG. 3b , thelower output of the amplifier 5 is shared by i) the upper input of thespeaker 3 and ii) the upper input of the speaker 2, and the upper outputof the amplifier 6 is shared by i) the lower input of the speaker 3 andii) the lower input of the speaker 1. At least one other configurationis possible.

In one embodiment of the invention, each output leg of the firstamplifier 5 and second amplifier 6 may exhibit a voltage swing that isthe sum of the amplitudes of x1, x2, and x3. This means that, ascompared to the conventional or brute force technique where each of x1,x2 and x3 has its respective full bridge (BTL-output) amplifier, eitherthe supply voltages have to be increased or the amplitude of one or moreof x1, x2, and x3 is decreased. Viewed differently, it is the sum of theamplitudes of the three channels x1, x2 and x3 that should be limited asthey appear at the output leg of the amplifier 5 or amplifier 6 (tomaintain linear behavior), and not necessarily each individual channel.As an example, if x1 and x2 are loud (and thus contain the majority ofthe playback power) while x3 is quiet, then the channel amplitudes of x1and x2 may not need to be limited as much as they would have been had x3been louder. Note however that for the same loads (impedances), anembodiment of the invention would not consume more power than the bruteforce technique; it may however need to have either a) a larger supplyvoltage, such as three times the supply voltage of the brute forcetechnique, or b) a sufficient reduction in the amplitudes of any one ormore of the inputs x1, x2 and x3, such as by reducing the amplitudes ofx1 and x2 but not that of x3 (in order to keep the same supply voltage).

It should also be noted that, in one embodiment of the invention, eachof the first and second amplifiers 5, 6 has common mode outputcapability. For example, if x1=0 (e.g., the channel is muted), then theaverage voltage of the two output legs of the amplifier Al will be thesum of x2+x3 which is non-zero since x2 and x3 are independent audiochannels.

Turning now to FIG. 4, this is a combined block diagram and circuitschematic of the audio amplifier circuit 4 that has been implementedusing four half bridge output stages 8, 9, 10, 11. More particularly,the first amplifier 5 (see any one of FIG. 1-FIG. 3b ) is implementedusing a first half bridge output stage 8 and a first input stage 12,where the latter is a summing and differencing amplifier circuit that isto receive the three audio channel signals x1, x2, x3, and provides afirst combination thereof to an input of the first half bridge outputstage 8 as shown (referenced as a signal Vaudio). The first half bridgeoutput stage 8 has a single ended output, which provides the combinationof x1, x2, x3 as Vaudio, defined by the first input stage 12, relativeto ground. The second leg of the first amplifier 5 is provided by asingle ended output of a second half bridge output stage 9, where thelatter has an input that is fed by a second input stage 13. The latteris another summing and differencing amplifier circuit that provides asecond, different combination of the three audio channel signals x1, x2,x3. The half bridge output stages 8, 9 provide power gain to drive aspeaker load (in this case the input terminals of the speaker 1), whilethe summing and differencing amplifier circuit (within the input stages12, 13) need not provide power gain but may be simply voltage signalconditioning circuits.

Referring back to FIG. 1-FIG. 3b , the audio amplifier circuit 4 alsohas the second amplifier 6, and in FIG. 4 this may be implemented byanother pair of half bridge output stage and input stage circuits,namely an input stage 14 (e.g., a summing and differencing amplifiercircuit) that feeds an input of a third half bridge output stage 10,while a fourth input stage 15 feeds the input of a fourth half bridgeoutput stage 11. Each output stage 10, 11 may have a single ended outputand may also have a single ended input that receives Vaudio from itsrespective input stage 14, 15.

Note that the configuration of FIG. 4, including the combination of theaudio channels x1, x2, x3 that is assigned to each output leg of theaudio amplifier circuit 4, is the same as in FIG. 1. The approach ofFIG. 4 in which a separate input stage 12, 13, 14, 15 is used to feed aseparate half bridge output stage 8, 9, 10, 11 can also be used toimplement different configurations of the audio amplifier circuit 4,including those depicted in FIG. 2, FIG. 3a , and FIG. 3 b.

A combined block diagram and circuit schematic of an example half bridgeoutput stage 8, 9, 10, or 11 is illustrated in FIG. 5. This is asimplified block diagram of a pulse width modulation based half bridgeoutput stage, which is a class D amplifier. Pulse width modulation (PWM)is achieved using a combination of a triangle wave oscillator 20 feedingan input of a comparator 19, while the other input of the comparator 19receives the audio signal Vaudio. The power transistors 17, 18 aredriven by a power transistor driver circuit 22, which responds to thePWM modulated signal by switching on and switching off the powertransistors 17, 18 accordingly. The transistors 17, 18 switch an outputnode alternately to either Vdd or Vss, while the output node voltage isbeing filtered by a suitable L-C filter stage, having inductorL_(filter) and shunt capacitor C_(filter) as shown. Although not shown,more complex circuit arrangements may be used for the half bridge outputstage 8, 9, 10 or 11, and these may include additional circuitry such asfeedback circuitry to help regulate the output node voltage, and afilterless class D modulator circuit (which may avoid the need for theL-C filter stage at the output node.) Other half bridge output stagedesigns are possible, including class D types that use pulse densitymodulation (PDM) and those that are not class D types.

The summing and differencing amplifier circuit in the input stage 12,13, 14, or 15 may include an op-amp based analog summing circuit and anop-amp based differential amplifier (or analog substractor). These wouldoperate upon the audio channel signals x1, x2, x3 in the time domain andin analog form, to produce an analog combination of the three audiochannel signals at the signal input of a respective half bridge outputstage 8, 9, 10, or 11. Alternatively, the summing and differencingamplifier circuit of each input stage 12, 13, 14, or 15 may have adigital signal processor that performs summing and subtraction upon theaudio channel signals x1, x2, x3 in digital form (e.g., in the discretetime domain) to produce a digital form of the combination of the threeaudio channel signals (being the audio signal Vaudio). Adigital-to-analog converter that converts the digital form of Vaudiointo analog form, at the signal input of one of the half bridge outputstages 8, 9, 10, 11 may then be needed, to interface with the examplehalf bridge output stage depicted in FIG. 5 where Vaudio in that case isan analog voltage signal.

The audio amplifier circuit 4 described above in its various embodimentsprovides a solution to the problem of driving three separate speakerloads with three audio channels (all of which may be independent of oneanother), and doing so in a manner that may reduce the total count ofpower transistors. The audio amplifier circuit 4 may be used indifferent types of consumer electronic audio devices. One such devicethat may benefit specially from the approaches described above is aportable electronic audio device, such as a laptop, smartphone, tablet,portable speaker, or speaker dock which may be a high volume manufactureitem that has limited volume within its housing 26 (thereby motivating acircuit design that can reduce the number of large and expensivecomponents such as power transistors). FIG. 6 depicts a block diagram ofrelevant parts of an example portable electronic audio device, includingits housing 26 in which the audio amplifier circuit 4 can beimplemented. The housing 26 may be that of a smartphone, laptop, tabletcomputer, or a speaker dock, to name a few examples. In those instances,the speakers 1, 2, 3 are integrated within the housing 26. In otherembodiments, one or more of the speakers 1, 2, 3 may be externalspeakers, that is external to the housing 26, which are connected to theoutput legs of the audio amplifier circuit 4 through any suitable audiopower cable and connector mechanism.

The inputs (or input signals) to the audio amplifier circuit 4, whichinclude the three audio channel signals x1, x2, x3, may be obtained fromcommunications circuitry 27 which receives a single channel audio signalor a multi-channel audio signal through downlink communications, from aremote music or video file server 25 (e.g., a music or video stream overthe Internet). Alternatively, a single channel or multi-channel audiosignal may be obtained from a phone 24 of a far-end user that is engagedin a two-way voice communications session with a near-end user (notshown) of the housing 26. The session in that case may be genericallyreferred to as a voice call or it may be a video call. Anotherpossibility for sourcing the multi-channel audio signal (which containsat least x1, x2, x3) is by a processor 32 reading a music or video filethat is stored in a local media file storage 30, within the housing 26for example. The processor 32 may be programmed in accordance with anoperating system and any one or more application programs (e.g., app1,app2), which are stored in program storage 33 that is also locatedwithin the housing 26. In such instances, a multi-channel audio signalx1, x2, x3 (optionally having additional audio channels) may be providedin discrete time domain to the audio amplifier circuit 4, during eitherplayback or during a call, in order to produce sound through thespeakers 1, 2, 3. Details concerning how the three audio channels x1,x2, x3 are processed by the audio amplifier circuit 4 may be inaccordance with any of the embodiments described above in connectionwith FIG. 1-FIG. 5.

While certain embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat the invention is not limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those of ordinary skill in the art. For instance, the halfbridge output stage depicted in FIG. 5 may have positive and negativevoltage supply inputs (Vdd is positive and Vss is negative), but asingle ended voltage supply is also possible (where Vdd is positive andVss is ground or zero volts.) The description is thus to be regarded asillustrative instead of limiting.

What is claimed is:
 1. An audio amplifier circuit that is to receivethree audio channel signals as inputs, comprising: a first amplifierhaving a first pair of outputs, wherein one of the first pair of outputsis to produce a first combination sum of the three audio channelsignals, and the other produces a second, different combination sum ofthe three audio channel signals; and a second amplifier having a secondpair of outputs, wherein one of the second pair of outputs produces athird, different combination sum of the three audio channel signals andthe other produces a fourth, different combination sum of the threeaudio channel signals.
 2. The audio amplifier circuit of claim 1 incombination with first, second and third speakers each having arespective pair of inputs, wherein the first pair of outputs and thesecond pair of outputs are coupled to drive the respective pairs ofinputs of the first speaker, the second speaker, and the third speakersuch that each of two outputs, of the first and second pairs of outputs,is shared by inputs of two of the first, second and third speakers. 3.The audio amplifier circuit of claim 1 in combination with first, secondand third speakers each having a respective pair of inputs, wherein thefirst pair of outputs is coupled to drive the respective pair of inputsof the first speaker, respectively the second pair of outputs is coupledto drive the respective pair of inputs of the second speaker,respectively, and a) one of the respective pair of inputs of the thirdspeaker is coupled to be driven by one of the first pair of outputs, andb) the other one of the respective pair of inputs of the third speakeris coupled to be driven by one of the second pair of outputs.
 4. Theaudio amplifier circuit of claim 1 wherein each of the first and secondamplifiers comprises a full bridge class D output stage coupled to driveits speaker as a floating load.
 5. The audio amplifier circuit of claim4 in combination with first, second and third speakers each having arespective pair of inputs, wherein the first pair of outputs and thesecond pair of outputs are coupled to drive the respective pairs ofinputs of the first speaker, the second speaker, and the third speakersuch that each of two outputs, of the first and second pairs of outputs,is shared by inputs of two of the first, second and third speakers. 6.The audio amplifier circuit of claim 1 wherein each of the first andsecond amplifiers comprises: a first half bridge output stage; a firstinput stage being a summing and differencing amplifier circuit that isto receive the three audio channel signals and provide a firstcombination thereof to an input of the first half bridge output stage; asecond half bridge output stage; and a second input stage being asumming and differencing amplifier circuit that is to receive the threeaudio channel signals and provide a second, different combinationthereof to an input of the second half bridge output stage.
 7. The audioamplifier circuit of claim 6 in combination with first, second and thirdspeakers each having a respective pair of inputs, wherein the first pairof outputs and the second pair of outputs are coupled to drive therespective pairs of inputs of the first speaker, the second speaker, andthe third speaker such that each of two outputs, of the first and secondpairs of outputs, is shared by inputs of two of the first, second andthird speakers.
 8. An audio amplifier circuit that is to receive threeaudio channel signals, comprising: a first class D half-bridgeamplifier; a first input stage being a summing and differencingamplifier circuit that is to receive the three audio channel signals andprovide a first combination thereof to a signal input of the first halfbridge amplifier; a second class D half-bridge amplifier; a second inputstage being a summing and differencing amplifier circuit that is toreceive the three audio channel signals and provide a second combinationthereof, to a signal input of the second half bridge amplifier; a thirdclass D half-bridge amplifier; a third input stage being a summing anddifferencing amplifier circuit that is to receive the three audiochannel signals and provide a third combination thereof, to a signalinput of the third half bridge amplifier; a fourth class D half-bridgeamplifier; and a fourth input stage being a summing and differencingamplifier circuit that is to receive the three audio channel signals andprovide a fourth combination thereof, to a signal input of the fourthhalf bridge amplifier.
 9. The audio amplifier circuit of claim 8 incombination with first, second and third speakers each having arespective pair of drive inputs, wherein an output of the first halfbridge amplifier, an output of the second half bridge amplifier, anoutput of the third half bridge amplifier, and an output of the fourthhalf bridge amplifier are coupled to drive the respective pairs ofinputs of the first speaker, second speaker and third speaker such thateach of two outputs, of the first, second, third and fourth half bridgeamplifiers, is shared by drive inputs of two of the first, second andthird speakers.
 10. The audio amplifier circuit of claim 8 incombination with first, second and third speakers each having arespective pair of drive inputs, wherein an output of the first halfbridge amplifier and an output of the second half bridge amplifier arecoupled to drive the respective pair of inputs of the first speaker, anoutput of the third half bridge amplifier and an output of the fourthhalf bridge amplifier are coupled to drive the respective pair of inputsof the second speaker, and a) one of the respective pair of inputs ofthe third speaker is coupled to be driven by the output of the firsthalf bridge amplifier or the second half bridge amplifier, and b) theother one of the respective pair of inputs of the third speaker iscoupled to be driven by the output of the third half bridge amplifier orthe fourth half bridge amplifier.
 11. The audio amplifier circuit ofclaim 8 wherein the first half bridge amplifier has positive andnegative voltage supply inputs.
 12. The audio amplifier circuit of claim8 wherein the summing and differencing amplifier circuit comprises anop-amp based analog summing circuit and an op-amp based differentialamplifier or analog subtractor, which operate upon the audio channelsignals in analog form to produce an analog combination of the threeaudio channel signals at the signal input of one of the half bridgeamplifiers.
 13. The audio amplifier circuit of claim 8 wherein thesumming and differencing amplifier circuit comprises: a digital signalprocessor that performs summing and subtraction upon the audio channelsignals in digital form to produce a digital form of the combination ofthe three audio channel signals; and a digital to analog converter thatconverts the digital form into analog form at the signal input of one ofthe half bridge amplifiers.
 14. A portable electronic audio devicecomprising: a laptop, smartphone, tablet, or speaker dock housing havingintegrated therein first, second, and third speakers, and an audioamplifier circuit having three inputs to receive three audio channelsignals, respectively, the audio amplifier circuit having a firstamplifier having a first pair of outputs, wherein one of the first pairof outputs is to produce a first combination sum of the three audiochannel signals, and the other produces a second, different combinationsum of the three audio channel signals, and a second amplifier having asecond pair of outputs, wherein one of the second pair of outputsproduces a third, different combination sum of the three audio channelsignals and the other produces a fourth, different combination sum ofthe three audio channel signals, wherein each of the first, second andthird speakers has a respective pair of inputs, and wherein the firstpair of outputs and the second pair of outputs are coupled to drive therespective pairs of inputs of the first speaker, second speaker, andthird speaker such that each of two outputs, of the first and secondpairs of outputs, is shared by inputs of two of the first, second andthird speakers.
 15. The portable electronic audio device of claim 14wherein each of the first and second amplifiers comprises a class D fullbridge output stage coupled to drive at least two speaker inputs as afloating load.
 16. The portable electronic audio device of claim 14wherein each of the first and second amplifiers comprises: a first classD half bridge output stage; a first input stage being a summing anddifferencing amplifier circuit that is to receive the three audiochannel signals and provide a first combination thereof to an input ofthe first class D half bridge output stage; a second class D half bridgeoutput stage; and a second input stage being a summing and differencingamplifier circuit that is to receive the three audio channel signals andprovide a second, different combination thereof to an input of thesecond class D half bridge output stage.